This invention concerns a negative resist material used for manufacturing active elements, metallization patterns and the like involved in constructing semiconductor devices, a method of manufacturing such a negative resist material, and a method for forming resist patterns using this negative resist material.
In recent years, due to the higher degree of integration in semiconductor devices, a technique has come to be required in the manufacturing process which is capable of forming a fine resist pattern with a high aspect ratio (ratio of height of pattern/width of pattern). When carrying out multi-layer metallization, however, steps occur between the metallization formed on the substrate and the substrate surface, and these steps interfere when it is desired to form a new resist pattern for the next metallization.
In order to form a resist pattern on such a substrate with a high degree of dimensional precision, it is desirable to form a resist layer of sufficient thickness to cover these steps, thereby planarizing the surface following which the planarized surface can be exposed to electron beam or the like. By direct irradiation, however, it is extremely difficult in practice to form a pattern of the order of several submicrons in thickness on a film which may be as thick as, for example, 2 .mu.m. This is particularly due to the fact that, although the number of backscattered electrons can be reduced by making the film thicker, forward scattering or scattering of incident electrons within the film becomes more marked for thicker films.
To overcome the above disadvantages, a bi-layer resist process has been proposed by which a fine resist pattern with a high aspect ratio can be obtained even from a resist film of considerable thickness.
This bi-layer resist process will be briefly explained with reference to FIGS. 1A to 1C. These drawings show part of the wafer in sectional outline.
In the bi-layer resist process, a resist pattern is obtained by using a lower resist layer 15 consisting of a thick film to absorb step 13 on substrate 11 thereby planarizing the surface, and an upper resist layer 17 containing silicon which is formed on lower layer 15 (see FIG. 1A). This upper layer 17 normally has a film thickness of about 0.15-0.3 .mu.m, and is patterned by exposure to light or an electron beam, or other suitable means to obtain a resist pattern 17a (see FIG. 1B). The lower layer 15 is removed by O.sub.2 --RIE (Reactive Ion Etching) during which the resist pattern 17a is then used as an etching mask. In this way, the upper layer resist pattern 17a can be transferred to the lower layer 15, and a fine bi-layer resist pattern 19 with a high aspect ratio can thus be obtained (see FIG. 1C). Electron beam lithography using a bi-layer resist process can effectively reduce both the back scattering and forward scattering mentioned above.
Different resists have been proposed for this bi-layer process, and those used as the upper layer will now be described. As the upper layer must have resistance to O.sub.2 --RIE, various materials containing silicon were developed, for example that which is disclosed in "Journal of the Electrochemical Society, 130 [9], p. 1962 (1983)). The material proposed in this reference is a copolymer of trimethylsilylstyrene (denoted by SiSt) as a silicon-containing photoresist, and chloromethylstyrene (denoted by CMS), in a copolymerization ratio of 90:10. It is reported that the copolymer P (SiSt.sub.90 --CMS.sub.10) has both excellent O.sub.2 --RIE resistance and sensitivity to electron beams. According to this report, when P (SiSt.sub.90 --CMS.sub.10) is subjected to O.sub.2 --RIE, the amount of etching (which may hereafter be also refer to as the initial etching amount), which takes place until the decrease in film thickness shows the very small value of several A/min, is 220-290 .ANG.. Further, the sensitivity to electron beams (radiationdose at which gelation being, D.sub.G.sup.i.) is stated to be 2.1 .mu.C/cm.sup.2.
It is also stated that when P (siSt.sub.90 --CMS.sub.10) was used, a resist pattern of submicron order with thickness 1.6 .mu.m was formed when the dose was 4.5 .mu.C/cm.sup.2.
Conventional photoresists of the type described above present a problem in that, because the initial etching amount is as high as 220.degree.-290 .ANG., they do not permit much uch process latitude. More specifically, since the initial etching amount is so high when this resist is used as an etching mask, the resist film has to be deposited fairly thickly. If on the other hand the upper resist layer is too thick, it is no longer possible to obtain a fine resist pattern with a high aspect ratio. There is therefore a limit to the thickness of the upper layer, and for this reason, the lower resist layer also cannot be made too thick.
Another problem with conventional resists is that the actual dose required for formation of submicron pattern is 4.5 .mu.C/m.sup.2, which is high (meaning low sensitivity). Considering the fact that throughput with electron beam exposure is lower than with light exposure, a resist of higher sensitivity is required.
Many linear (or straight-chain) siloxanes consisting of silicone resins become insoluble in solvents when they are irradiated by an electron beam and, as they are also resistant to reactive ion etching with oxygen (O.sub.2 --RIE), various applications in bi-layer electron beam resists have been proposed. In cases where the molecular weight is not sufficiently high, however, many of these siloxanes approach the liquid state, so that their use as resists involves problems.
In this connection it is thought that "ladder" type siloxane resins, which are trifunctional silane polymers, may provide an answer to most of these problems on account of their special structure, wherein the two siloxane chains are fixed through an oxygen atom. Ladder structure siloxane resins are usually manufactured by polymerizing lower polymers obtained by the hydrolysis of trifunctional silane monomers with three functional groups, in the presence of an alkali catalyst, so as to give polymers of higher molecular weight.
It is, for example, widely known that phenyl trichlorosilane, which is one of the trifunctional silane monomers mentioned above, can be hydrolysed to give a oligomer, from which high molecular weight, benzene-soluble poly(phenylsilsesquioxane) can be obtained by heating in benzene in the presence of trace amounts of KOH as catalyst (J. Amer. Chem. Soc., Vol. 82, 1960, p. 6194-6195).
Silicone resin homopolymers or copolymers, where the substituent attached to silicon is allyl or chloromethyl, have properties of coatability and electron beam sensitivity which make them eminently suitable as bi-layer electron beam resists of the type the inventors have been considering. If the above method is applied to their synthesis, however, a gel is formed if the polymerization temperature is higher than 150.degree. C., and it is difficult to obtain a polymer which is soluble in solvents. Further, even if the polymerization is carried out at a lower temperature, part is still converted to gel with the result that most of the siloxane extracted by solvent consists of lower molecular weight polymers.